From the past, radio receiving device which receive broadcast radio waves and reproduce their broadcast contents have been widespread. Although traditionally, as such radio broadcasts, analog broadcasts have been employed, the use of digital broadcasts has advanced in recent years.
One digital broadcasting system is the IBOC (In-Band On-Channel) system, which is employed in the USA. In this IBOC system, hybrid broadcasting is employed, in which a digital broadcast can be appended to an analog broadcast. By doing this, it becomes possible to enhance the sound quality as a result of HD (High Definition) radio broadcasting in which digital broadcasting is employed. In a hybrid broadcast according to this IBOC system, the digital broadcast carrier wave is located in the frequency bands adjacent to the analog broadcast carrier wave, on both its sides.
Now, it is not the case that all broadcasting stations perform hybrid broadcasting. In other words, although hybrid broadcasting may be performed by some broadcasting stations, other broadcasting stations may only perform analog broadcasting. In such a case, for radio broadcasting by the amplitude modulation method, a technique has been proposed of performing auto seek in order to find a broadcasting station which is performing hybrid broadcasting (refer to Patent Document #1, hereinafter termed the “prior art example”).
In this prior art example, during auto seek, the system starts from an auto seek start frequency, and, as tuning frequencies, the tuner is sequentially set to frequencies spaced apart by some predetermined frequency interval. And, each time a tuning frequency is set, the electric field intensity of the radio wave which corresponds to that tuning frequency is detected by detecting signal power having the bandwidth of a normal analog broadcast.
Next, from the result of detection of the electric field intensity, a judgment is made as to whether a broadcast wave corresponding to either an analog broadcast or a hybrid broadcast, either of which have a chosen frequency by dialing, is being received. If the result of this judgment is negative, then no decision is made as to whether or not this is a hybrid broadcast, but instead the tuning frequency is changed over.
On the other hand, if the result of this judgment as to whether a broadcast wave corresponding to either an analog broadcast or a hybrid broadcast is being received, either of which have a chosen frequency by dialing, is affirmative, then the received signal is digitally analyzed, and a judgment is made as to whether or not it includes a digital broadcast. As a result, auto seeking for a hybrid broadcast is implemented more rapidly than in the case of, each time a tuning frequency is set, digitally analyzing the received signal and making a judgment as to whether or not a digital broadcast is included.    Patent Document #1: Japanese Laid-Open Patent Publication 2005-12593.